X-ray apparatus



New I9, 1949. ,muwmxa ETAL 25222536 Patented Nov. 19, 1940 UNITED STATES PATENT OFFICE X-RAY APPARATUS Alfred Kuntke, Hamburg,

Wilhelm Daumann,

Germany, and Carl Eindhoven, Netherlands,

assignors, by mesne assignments, Ato Hartford National Bank and Trust Company,

Conn., as trustee Hartford,

10 Claims.

Our invention relates to X-ray apparatus .of the type in which energy is `sup-plied tothe X-ray tube by a previously-charged condenser.

With such apparatus, the condenser is charged to a predetermined voltage from an A. C. supply with a low charging current which vflows during numerous cycles of the supply voltage, and `it is possible to take instantaneousradiographs with very high Vpower without overloading the supply mains. The duration Aof the active discharge through the tube depends -upon the initial voltage of the condenser and the intensity of the filament-heating current of the tube, and the discharge current through the tube may be many times greater lthan the charging current. Thus, it is possible to take a radiographof very short duration with a `current which is large and is limited only by the Acapacity ofthe X-ray tube.

Such apparatus, however, have the disadvantage that the condenserr voltage, and lthus the operating voltage, of the X-ray tube decreases during the discharge in accordance with Aa potential curve whose shape is unfavorable for the efcient utilization of the energy ystored Within the condenser. It has been proposed to improve the shape oivthis potential curve by Vusing a second accumulator of energy, such as a choke coil, which is connected in series with the condenser. This choke is given such an inductance that it will absorb part of the voltage at the beginning of the discharge, and accumulate a part of the energy output of the condenser with gradually decreasing wattage to return this energy to the Y-ray tube after the tube voltage has increased to a value equal to the condenser voltage. As a result, the curve of the voltage across the X-ray tube has a convex shape in which the central portion slowly rises to and recedes from a ymaximum value to form a portion in which the voltage is substantially constant.

Although this choke coil improves the shape of the curve of the discharge current and voltage through the tube, it cannot in any case bring about a sufficiently high degree of invariability of the voltage taken over the duration of the exposure.

The main object of the present invention is to make the terminal voltage of the X-ray tube, during a considerable portion of the exposure time, substantially independent of the variation in terminal voltage of the condenser.

Another object of the invention is to decrease the necessary value of the capacity of the condenser supplying, the X-ray tube with current.

A further object of the invention is 'GQ ely control the exposure time and the X-ray energy in an X-ray installation operating with a condenser discharge.

A still further object of the rinvention is to adjust the terminal voltage of an X-ray tube enerf gized by a charged condenser to a predetermined value which is independent of the voltage to which the condenser is vcharged up.

A still further object of the invention is to more efficiently utilize the energy stored within the condensers of such apparatus. v

Further objects of the invention will appear as the description progresses. f

In accordance with the invention we connect in series with the X-ray tube an impedance which, in contradistinction kto the choke coil, is an energy consumer and has a variable resistance. This impedance absorbs a portion of the condenser voltage and also consumes a part of the energy stored within the condenser, and its resistance is so varied relatively toy lthe condenser voltage, that the absorbedvoltage decreases in such a manner that the voltage across the X-ray tube remains substantially constant as long as the condenser voltage exceeds a given value.

The energy consumer mayconsist of a conductor having a vvariable resistance which dcpends on the voltage applied to it, so that when it is connected in series with a constant ohrnic resistance it keeps the current constant. Conductors like these are generally known as variators. n

'Wevprefeig however, to use as an energy con- Isumer a grid-controlled discharge tubehereinafter referred to as a control tube-because the resistance of such a tube can be readily controlled in an inertialess manner and the tube may also be used to control the discharge current through the X-ray tube. The potential variations at the X-ray tube are transmitted in an amplified manner by the control tube-Which lacts as a direct current amplier-to that part of the circuit which is connected in series with the X-ray, i. e., when the condenser voltage decreases the voltageat the control tube also decreases, but the voltage across the X-ray tube decreases only slightly.

For the above purpose, the grid of the control tube is preferably connected through a source of constant positive bias to a point whose potential relatively to the cathode ofthe control tube is negative and varies in the'same man,- ner as the voltage across the X-ray tube. Thus, by making the positive bias and/or the ratiol between the negative supplemental voltage and tube voltage adjustable, it is possible to select at will,l Within Wide limits, the Voltage at the X-ray tube.

We prefer to charge the condenser to a voltage Which is about twice the voltage across the X-ray tube, because this gives the. optimum ratio l,Ice-

tween the'` energy consumed in the AXA-ray `tube and that accumulatedl in the condenser.V Thus, the voltageto which the condenser is charged should be twice the mean value of desired range" of Voltage to be used across the X-ray tube.:

In order to more clearly set forth the inven-v Figs. 4 and 5 are schematic diagrams ci -ray apparatus embodying the invention. v, A, As stated above, if a condenser is lallowed to discharge directly through an X-raytube, the

Voltage of the condenser, as Well as the Avoltage across the X-ray tube, will decrease vduring the discharge. More particularly, as the chargerQ `of a condenser, having a capacity C decreases, the voltage V across the condenser-'r-Which'is related to the charge bythe equation i also decreases. If the voltage to which the vcondenser is charged is E, the voltage V during the discharge throughiaxed resistance-R varieswiththe time'tL in accordance with the equation 1' .t V= Ele-CE l This is illustrated in Fig. 1 in which the abscissae represent time, the ordinates'represent the voltage across the condenser, and reference numeral 28V indicates the voltage-time curve ofiy the condenser. A line EP tangent ,to curve 28l at apoint E intercepts the abscissaaxis ina point?.

To maintain the voltage Within .certainlimits during the radiographic exposure, the capaciti7 ofthe condenser may be increased. VThus curve 28 willA become flatter and line OP will vintercept the abscissa 'axis further to the right, Vand the voltage Will decrease more-slowly. Thus, -the smaller are to be the voltage variations -permitted during an exposure of a given duration and Awith a given number of milliampere-seconds, the larger' must -be the capacitycf the condenser.

` `,In any case only a relatively. small part of the The ratio-between the condenservvoltageand.the

energy stored Within the condenser is effective in producing useful X-rays, rand the major `part of the energy remains in the condenser if the tube is disconnected from the circuit, or is lost if the tube remains connected. t'

In Fig. 2 theordinates represent the voltage V across .the condenser and the Vabscissae represent the numberof milliampere-seconds passing through the X-raytube. If .Q1 is `the charge of the condenser at the beginning of the discharge,

the charge present after a time Tis Q=C'V=Q1'v ,'l'idtV milliampere-seconds is consequently'substantially linear as indicated by line'SU. .y i v If,v in accordance ,with the inventiomthe tube 1 voltage lER is made constant and equal to CE1',

1 the area of rectanglefCEiX1M1 represents the energy in Watt-seconds consumed'by'the tube.

If CE1 equals x/2OS, as is the' case in Fig.` 2, this energy is equal to 1/2 the energy content of the condenser. A parabolic curve, which deviates only slightly from a semi-circle having a center M1 and a radius CM1, representsI the energy availablein the condenser (vfidt) as a functiori'of the milliampere-second's, and the ordinates ofthe points on the curve lineare a measure of n the joules.

If, for the given initial condenser voltage CS, i va tube voltage CE2 is selected, the area of rectangle OE2X2M2 represents the useful energy available at a constant tube voltage CE2, and if ,a tube voltage CEa is selected, the useful energy is represented by the area of rectangle OEaXaMa.

The ordinates 1VI2N2 and M3N3 for f idt equal -to CM2 and CM3 respectively are proportional to the'areas of rectangles CEzXzMz and CEaXsMs respectively. In the Fig. 2 MzNz is chosen vequal to MaNs, to show that at the different tube Voltages E2 and E3, radiographs may be taken with the same number of Watt-seconds.

When the tube voltage is CE1 and the current consumer according to the invention connected in series with the condenser absorbs the re maining voltage, the energy represented by the area of triangle E1X1S is consumed in the seriesconnected consumer and the energy represented by rectangle CE1X1M1 is consumed in the X-ray tube. If the tube is cut out of circuit in time, the energy represented by triangle X1M1U remains in the condenser, and thus for the next eX- posure it is necessary to supply to the condenser only the energy represented bythe area of figure CM1X1S.

The' advantages as regards eiciency of an apparatus according to the invention over an apparatus in which a current-controlling consumer is not used, Will be pointed out with referenceto Fig. 3 in which the abscissae represent milliampere-seconds and the ordinates represent the condenser Voltage. Assuming that, in an apparatus without a current consumer, the voltage across the tube must not decrease more than 20% during the exposure, it is possible to use a condenser charged to a voltage CG and having a capacity C equal to of the total energy.

When using a current consumer according to the invention, the condenser may be given a capacity C2 which is much lower than C1 and, as indicated in Fig. 3, is equal to This condenser is charged to a higher voltage shown as OA, which is equal to CG-l-HK. n If the tube voltage isI adjusted to a value CF equal to 1/2CA, i. e., so that it has the same average value as in the rst case, about 50% (rectangle CFDK) ofthe energy stored in condenser Cz (triangle CAB) is eiciently utilized, Whereas the number of watt-seconds is the same in the first case, and thisris obtained With 1/5 of the capacity of condenser C1.

As a specic example,assume that condenser C2 is giver; a capacity of 4.aF. and the voltage CA is given a value of 100 kv. In this case the total energy of the condenser will be 20 kw.-sec. of which l kw.-sec. will be efliciently consumed at a tube voltage of 50 kv. To obtain the same energy by means of a voltage which is not maintained constant by the means according to the invention and whose'initial value is 55 kv. and final value is 45 kv., the condenser would have to have a capacity of 20aF and 28 kw.-sec:iwould have to be accumulated therein.

From the above it appears that an apparatus according to invention has numerous advantages over prior art condenser apparatus. For example, the tube voltage is more favorable because it decreases to only a slight degree during exposure, the energy to be supplied from the mains may be lower, and it is possible to take exposures of a given number of watt-seconds with a condenser of much lower capacity. Moreover the voltage across the tube may be regulated by adjusting the characteristic of the series-connected current consumer.

During the condenser discharge the chargingup device may be disconnected from the discharge tube, and Figures 2 and 3 refer to such a case.

Referring to Fig. 4 which shows the main parts of a circuit diagram for an X-ray apparatus of thisl kind, an X-ray tube 4| has its anode 42 connected through the triode 43 to one terminal of condenser 44. The cathode 45 of the X-ray tube 4I may be connected through switch device 46 with the other terminal of condenser 44. Switch device 45 is controlled by a magnet winding 41 and may be of the type described in the U. S. patent specification 1,946,329 to Alfred Kuntke of February 6, 1934. A transformer having a low voltage primary winding 48 and a high voltage secondary winding 49 serves for charging up the condenser 44 to a voltage considerably higher than the desired terminal voltage of X-ray tube 4I.

The primary winding 48 is energized by an A. C. supply when switch 6i! is closed. Condenser 44 is then charged up with direct current, because the current supplied by transformer 48, 49 to the condenser is rectified by the diode 50. Primary resistance I and the resistances of diode 5l) and transformer 48, 49 limit the current, so that only after a large number of cycles of the A. C. current, for example after a few seconds, the condenser voltage will have attained the desired value5 which is slightly below the maximum noload voltage of secondary winding 49.

Magnet winding 41 is supplied with current from the A. C. network, to which it is connected through switch 6i). When energized it keeps switch device 45 open.

In shunt with X-ray tube 4| is a resistance 52 having a tap 53. The latter is connected through resistance 54 and battery 55 to the grid 56 of triode 5I). The anode 51 of the triode is connected to transformer winding 49 and the cathode 53 is connected to the anode 42 of X-ray tube 4I. Battery 55 imparts a positive bias to grid 5S.

After the condenser 44 is duly charged up, switch 6l) is opened, whereby energization of transformer 4I), 49 ceases. At the same time magnet winding 4'I is deenergized so that it no longer keeps switch device open and the discharge current circuit is established. Current flows now from condenser 44 through triode 43 to X-ray tube 4| and from there through switch device 45 back to condenser 44. Triode v43 acts therewith as a control tube. Tubes 43 and 50 are preferably incandescent cathode tubes. The heating circuits for the Cathodes of tubes 4|, 43 and 5l) are omitted for simplicity.

During the discharge the condenser voltage falls rapidly and thus the voltage at the X-ray tube decreases. However the grid voltage of the control tube 43 becomes slightly less negative, and when the condenser voltage V drops to a value only slightly higher than the tube Voltage En, the conductivity of the control tube starts to increase appreciably; the voltage still present being practically entirely in favor of the X-ray tube. It is recommendable not to use this part of the condenser discharge for the exposure, because in this case the voltage at the X-ray tube falls quickly to a value which is low enough for the tube to become inactive as a source of X-rays. For this reason the tube should be made inactive previously, for example by eliminating the positive bias of grid 55. For this purpose a switch 59 may be provided by which battery 55 and resistance 54 may be shunted.

If desired, the charging-up device may remain connected and the internal resistance of the X- ray tube may be increased in the manner set forth in the U. S. patent application Ser. No. 11,355 to Albert Bouwers filed March 15, 1935, now Patent No. 2,157,474. In such cases the voltage drop in the supply device should remain low enough for the voltage at the tube to suice for the emission of X-rays in the condition of equilibrium set up when the condenser obtains in each cycle of the alternating current supply a charge which is equal to the chargeV delivered from the condenser to the X-ray tube.

If the internal resistance of the X-ray tube is not increased during the condenser discharge, there will be generally set une-after a large part of the energy accumulated in the condenser has been consumed-a condition of equilibrium in which the voltage across the tube is too low for the emission of rays. This results from the fact that the condenser is used so that it may be possible to use a comparatively feeble supply device which itself, in the case of a load having a low resistance, consumes by its impedance the Amajor part of the produced electromotive force. In this case it is not necessary tocut out the current quickly, it being assumed that the charging device delays the short-circuit-like low-resistance load of the X-ray tube and the load of lthe mains is not made too intense.

Fig. 5 illustrates the circuit arrangement of an X-ray apparatus according to the invention. in which the supply device remains connected to the X-ray tube during the discharge of the condenser. The apparatus comprises a high-tension supply transformer A having a primary winding 3E) connected to a suitable A. C. supply, v

and a high-tension secondary winding I. Winding I has one end connected to one electrode of condensers 4 and 5, and its other end connected through resistances 3| and 32 and rectiiiers 2 and 3, which are preferably incandescible cathode tubes, to points 33 and 34 connected tov the other electrodes of condensers 4 and 5 respectively.

Connected in series across points 33 and 34 are an energy-consuming impedance in the form of a grid-control discharge tube I having an anode I8, a control grid I3 and a cathode il, a choke coil I0 which serves to protect tube I from disruptive discharges, and an X-ray tube having an anode 9 and a cathode I2. Cathodes 8 and I2 are of the incandescible type, and the heating circuits therefore are omitted for simplicity. Thus it is es, one comprising a source of constant positive bias I5 vand the contact I6 of a normally-closed relay I 9, and the other comprising a source of negative Vbias 20 and a high resistance 2I.

If we assume the X-ray tube voltage as E, the voltage between anode 9 and the point I1 as and the voltage of the source I5 as y, the grid voltage of tube 1 will be equal to It is thus possible to express the X-ray tube voltage by the following formula:

in which D is 1 amplification factor of tube 'I, n is a constant depending upon the adjustment of potentiometer II, and V is the voltagebetween the anode I8 rand cathode I2. If VD is made very small relative to g, E will be substantially constant. Generally, 11D will be small compared with 1, and E will be substantially proportional to n. The biasing voltage supplied by the source 20 may be regulable. By changing this voltage, the tube voltage E may be adjusted to a predetermined value. Assuming that the no-load voltage of transformer A is equal to 0.5V1, the voltage V between anode I8 and cathode I2, with which the load starts, will be approximately equal to V1. A part ER thereof is in favor of the X-ray tube, which partis, for example, 50% of V1. v 'What hap-pens to the voltages after the discharge current is established, depends on the value at which the internal resistance of the X-rayitube is adjusted. 'If this resistance is sufficiently high, a condition of equilibriumis set up in which the condenser voltageV always exceeds ER, and the voltage at the X-ray tube Eremains about equal to ER. This may be made effective for taking radioscopes or radiographs of prolonged duration with-low power and a voltage ER that may be adjusted at will.

If, however, the current through the X-ray tube is adjusted to a value at which the poten-` tial drop in the charging circuit exceeds the difference between V1 and ER, as long as the con# denser voltage is higher than ER a larger amount of watts 'will be supplied by the condensers to the tube than is supplied to them by the charging device and the condenser voltage falls. As long as V remains considerably higher than ER, voltage E remains substantially constant. The more the dilerence between V and E decreases, the greater is the extent to which also E decreases. Due to this the negative grid bias obtained from the potentiometer I I becomes considerably smaller and the conductivity of tube 'I becomes higher, so that at last the voltage E falls back with the condenser voltage and recedes to ,al

value below ER, and the higher is the-adjusted current strength of the X-ray tube, the lower willbe this value which may be insuiiicient for the emission of -X-rays to occur.

In the above equation, the voltage g is a relatively-large percentage, for instance 10% of the voltage V. On the other hand D is only a fraction of. 1, with the yresult that VD may be small with respect to g, although V is larger than g.

In `the specific example g may equal 10,000 volts, D may equal 0.005 and 11. may equal 5. In this case if V varies `from 100,000 volts to 60,000 volts, substitution in the vabove equation showsl that the initial voltage E of the X-ray tube equals value of or 50,250 volts. Thus, the voltage E decreases only about 2%, i. e., from 51,200 volts to 50,250 volts whereas the condenser voltage V decreases b-y about 40%.

rRelay I9 serves tointerrupt the iiow of current through the X-ray tube, and may be controlled by an automatic device (not shown), for example, a time switch or a milliampere-second relay.' As soon as the contact I6 is opened, the grid I3, which is connected through the source of negative bias 20 and high resistance 2l to the tap I7, becomes negative, and the control tube 'I becomes non-conductive and interrupts the current through the X-ray tube. The number of milliarnpere-seconds may thus be limited at Will. V

Rectiiiers 2 and 3 may preferably be used to protect the A. C. supply mains or transformer A from being overloaded by limiting the charging currentA by their saturation current. For this-purpose the incandescible cathodes of vthese rectifiers are' so heated that even if condensers d and 5 are short-circuited or in adischarged condition they will absorb a current which is no higher than that corresponding to the highest permissible Vloads ofthe mains or the transformer; y.In this case, it may be desirable that the rectiiiers be surrounded by an X-ray-absorbingscreen', since inter alia the voltage at the valvesirnay increase to a A value at which appreciable radiation occurs, when the point of saturation is reached.

To obtain directly from the main supply loads a current-load exceeding that which can be supplied by the charging device at an eicient tube voltage as well as an eiicient current supply, the

.resistance ofthe X-ray tube may be increased during the discharging of the condenser in the manner describedl in the above mentioned U, S. patent" to Albert Bouwers, and the equilibrium may thus be displaced to a higher voltage at which serviceable X-rays are produced.

While we have described our invention in connection with specific examples and applications, we do not wish to be limited thereto, but desire the appendedclaims .to be construed as broadly as is permissible in view of theprior art.

What we claim is:

1. An X-ray'apparatus comprising an X-ray tube,` a condenser for storingenergy to be supplied to said tube, means including a supply source forr` charging said condenser to a predetermined voltage, an energy consumer of variable ohmic resistance connected in series between said X-ray tube and condenser, and means associated with said energy consumer and said X-ray tube for decreasing the ohmic resistance of the energy consumer with decreasing voltage across the condenser to thereby maintain the .voltage across the X-ray tube at a substantially constant value as long as the voltage across the condenser exceeds a given value.

2. An X-ray apparatus comprising an X-ray tube, a condenser for storing energy to be supplied to said tube, means including a supply source for charging said condenser to a predetermined voltage, a grid-controlled discharge tube connected in series between said X-ray tube and condenser and having a cathode, and means to automatically vary the potential difference between the grid and cathode in accordance with the decrease in the condenser voltage to thereby decrease the impedance of said tube to maintain the voltage across the X-ray tube at a substantially constant value as long as the voltage across the condenser exceeds a given value.

3. An X-ray apparatus comprising an X-ray tube, a condenser for storing energy to be supplied to said tube, meansincluding a supplysource for charging said condenser to a predetermined voltage, a discharge tube connected in series between said X-ray tube and condenser and having a control grid and a cathode, a grid circuit for said discharge tube including a source of constant voltage connected to raise the potential of the grid, and means associated with said X-ray tube and including a portion of said grid circuit to set up -in said circuit a voltage which lowers the potential of the grid and varies in the same sense as the voltage across the X-ray tube.

4. An X-ray apparatus comprising an Y-ray tube, a condenser for storing energy to be supplied to said tube, means including a supply source for charging said condenser to a predetermined voltage, a discharge tube connected in series between said condenser and the anode of the X-ray tube and having a control grid and a cathode, a grid circuit for said discharge tube including a source of positive grid bias, and means associated with said circuit to produce therein a source of negative grid bias, said latter means including a potentiometer connected across said X-ray tube.

5. An X-ray apparatus comprising an X-ray tube, a condenser for storing energy to be supplied to said tube, means including a supply source for charging said condenser to a predetermined voltage, a discharge tube connected in series between said X-ray tube and condenser and having a control grid and a cathode, a grid circuit for said discharge tube including said grid and said cathode, a source of constant positive bias and a second source of potential in said grid circuit, means associated with said X-ray tube to set up in said grid circuit an electromotive force derived from the voltage across the X-ray tube and varying therewith in the same sense and negatively biasing the grid, and means to make the latter source operative to further negatively bias the grid to counteract the constant potential source and prevent energization of the X-ray tube by the condenser.

6. An X-ray apparatus comprising an X-ray tube, a condenser for storing energy to be supplied to said tube, means including a supply source for charging said condenser to a predetermined voltage, a discharge tube connected in series between said X-ray tube and condenser and having a control. grid and a cathode, a grid circuit for said discharge tube including said grid, said cathode and a source of constant positive grid bias, and adjustable means associated with said X-ray tube and said discharge tube to set up in said circuit a negative grid bias which varies in the same sense as the voltage across the X-ray tube.

'7. An X-ray apparatus comprising an X-ray tube, a condenser for storing energy to be supplied to said tube, means for charging said condenser to a voltage about double the mean value of the range of voltage to be applied across the X-ray tube and including a supply source whose no-load voltage is substantially the same as said first voltage, av discharge tube connected in series between said X-ray tube and condenser and having a control grid and a cathode, a grid circuit for said discharge tube including said grid, said cathode, and a source of constant positive grid bias, and means associated with said X-ray tube and said circuit to set up in said circuit a source of negative grid bias derived from the voltage across the X-ray tube and varying in the same sense as said latter voltage.

8. An X-ray apparatus comprising an X-ray tube, a condenser for storing the energy to be supplied to said tube, means for charging said condenser to a predetermined voltage including a supply of A. C. voltage and a rectiiier tubev whose saturation current limits the maximum value of the charging current, a discharge circuit for said condenser and including said X-ray tube and an energy consumer of variable impedance connected in series with said X-:ray tube and condenser, and means associated with said energy consumer and connected with said discharge circuit to automatically decrease the impedance of said energy consumer in substantially direct proportion with the decrease in the condenser Voltage as long as said latter voltage exceeds a given value.

9. A method of energizing an X-ray tube from a previously-charged condenser comprising the steps of, discharging the condenser through the X-ray tube while absorbing a part'oi the condenser voltage in an energy consumer of variable ohmic resistance disposed between the X-ray tube and condenser, and decreasing the ohmic resistance ofthe energy consumer to maintain the voltage across the X-ray tube substantially constant as long as the condenser voltage exceeds a given value.

10. An X-ray apparatus comprising an X-ray tube having a cathode and an anode, a condenser for storing the energy to be supplied to said X-ray tube, means for charging the condenser to a predetermined voltage, a discharge circuit for said condenser and including said X-ray tube and a discharge tube having an anode, a cathode and a control grid, said discharge tube being connected in series with said X-ray tube with the cathode of the discharge tube connected to the anode of the X-ray tube, a resistance connected between the anode and cathode of the X-ray tube and having an adjustable tap, an electrical connection between said tap and said control grid, and a source of positive grid bias in said connection.

ALFRED KUNTKE. CARL WILHELM DAUMANN. 

